Abstract: To address the issue of transient impacts during switching between traditional grid-connected photovoltaic inverters and off-grid modes due to differing control strategies, this paper proposes a seamless switching control strategy for inverter parallel systems. The parallel inverter system employs droop control in both islanded and grid-connected modes, ensuring consistency of the control strategy across different operating modes. During the switch from islanded to grid-connected mode, a pre-synchronization control strategy is introduced to reduce the transient impact of mode switching. Finally, experiments verify that the proposed control algorithm can achieve seamless switching between the two modes.
1 Introduction
Parallel inverters typically employ centralized control, master-slave control, or distributed logic-based control. All of these methods rely on interconnecting cables, requiring communication lines between modules, increasing system power consumption, and reducing system reliability and scalability. Parallel inverters without interconnecting cables often utilize droop control. This parallel connection method enables long-distance inverter interconnection. Simultaneously, it avoids system complexity and high power consumption, improving system redundancy, reliability, and scalability.
Microgrid operating mode switching is the process of transitioning between islanded and grid-connected operating states. Traditional control methods use droop control for off-grid operation and PQ control for grid-connected operation, enabling stable operation of the microgrid system in both modes. However, the main inverter operates under current-based control in grid-connected mode and voltage-based control in islanded mode. This direct and abrupt mode switching inevitably generates transient impacts. Voltage-current weighted control strategies integrate these two methods during mode switching through weighting coefficients, effectively suppressing voltage and current overshoot. However, this method requires four controllers, which is cumbersome, and the weighting coefficients significantly affect control performance and system stability, making them difficult to determine.
This paper proposes a seamless switching control method for parallel inverter systems based on resistive droop. The droop equation is improved to achieve pre-synchronization between the parallel system and the power grid. A unified control strategy is adopted for both off-grid and grid-connected modes, achieving seamless switching between modes. Finally, experiments verify the effectiveness of the proposed control strategy.
2. Analysis of the structure and operating modes of the inverter parallel system
The equivalent circuit of two inverters connected in parallel is shown in Figure 1. U1 and U2 are the equivalent output voltages of the two inverters, φ1 and φ2 are the phase angle difference between the inverter output voltage and the load voltage, and R1 and R2 are the line resistances.
Therefore, the droop equation for the resistive line impedance can be obtained as follows:
The droop curve corresponding to the resistive droop equation is shown in Figure 2.
Under normal circumstances, the parallel system operates in grid-connected mode. When a grid fault occurs or its absorption capacity reaches saturation, the grid-connected system disconnects from the main grid and switches to islanded operation mode. At this time, the photovoltaic inverter system independently supplies power to the local load, ensuring power balance within the system and maintaining its own stable operation.
2.1 Isolated Operation Mode
Figure 3 shows the block diagram of the islanded operation mode of the inverter parallel system. In this diagram, Cdc is the DC bus capacitor, H is the full-bridge circuit, L and C are the filter inductor and capacitor, respectively, and RL is the local load.
By sampling the inverter output voltage uAC and output current iAC, the active power P and reactive power Q of the inverter output are obtained through the power calculation module. The inverter output voltage reference amplitude U and output frequency reference value f are obtained through the droop equation (1). The inverter output voltage reference is generated through a reference sine wave signal circuit. The inverter output is then controlled through voltage and current dual-loop regulation. The dual-loop regulation uses a quasi-PR controller, which can effectively suppress the influence of frequency fluctuations on the controller. Its expression is:
Where kpi and kri are the current loop ratio and resonant gain, respectively; kpv and krv are the voltage loop ratio and resonant gain, respectively; ωc is the center frequency bandwidth; and ω0 is the fundamental frequency.
2.2 Grid-connected operation mode
In grid-connected mode, the output voltage and phase of a parallel inverter are clamped by the PCC point, and the inverter needs to supply power to the grid. Power offset caused by grid fluctuations can be overcome by dynamically shifting the droop curve. The shifts of Un and fn are respectively...
By superimposing the translation amounts of the sag curves into the sag equation (1), we can obtain the grid sag equation.
2.3 Pre-synchronization Strategy
When the inverter switches from islanded mode to grid-connected mode, a pre-synchronization control strategy is introduced to ensure a smooth transition, as shown in Figure 4. The input to the pre-synchronization controller is the phase difference Δδ between the voltages across the static switch, and the output is added to the power loop to make the inverter output voltage vector track the grid voltage vector. The droop equation at this time is shown in equation (5). The static switch is closed immediately after pre-synchronization is completed. To avoid affecting the operating characteristics of the power loop in grid-connected mode, the pre-synchronization control output should be set to zero after the static switch is closed.
3. Experimental Results and Analysis
To verify the effectiveness of the control strategy proposed in this paper, a parallel system of two 1KVA, 50Hz single-phase inverters was established. The DSPTMS320F28335 manufactured by TI was used to realize the digital control of the system. The system parameters are shown in Table 1.
Figure 5 shows the waveforms from the islanded operation experiment of the parallel inverters, where u1 and u2 are the output voltages of the two inverters, and i1 and i2 are the output currents of the two inverters. The output currents i1 and i2 are approximately 2A, and the output power of each inverter is approximately 180W. The experimental results show that when the parallel inverters are in islanded operation, the output voltage and current of the inverters are basically the same, and they can automatically distribute the local load power evenly.
Figure 6 shows the pre-synchronization experimental waveforms when the parallel inverter switches from islanded operation mode to grid-connected operation mode. As can be seen from the figure, after the pre-synchronization strategy is activated, it takes approximately 8 power frequency cycles for the inverter output to achieve pre-synchronization with the grid. During the pre-synchronization process, the inverter output voltage gradually decreases in sync with the grid voltage, and there are no oscillations during the synchronization process. The experimental results show that the output voltage and phase of the parallel inverter can accurately track the grid, achieving grid-connected pre-synchronization.
Figure 7 shows the transient waveforms during the switching between islanded and grid-connected operation modes of the parallel inverter system. Figure 7(a) shows the transient waveform at the instant from islanded to grid-connected, and Figure 7(b) shows the transient waveform at the instant from grid-connected to islanded. As can be seen from the figures, the transition between islanded and grid-connected modes is smooth and without impact, with no overvoltage or overcurrent phenomena, ensuring continuous and uninterrupted power supply to the load. Furthermore, the inverter output voltage, inverter output current, and grid-connected current waveforms exhibit good quality. During stable operation in grid-connected mode, the output current of both inverters is approximately 8A, and the output power of each inverter is approximately 700W.
4. Conclusion
This paper proposes a seamless switching control strategy for a parallel photovoltaic inverter system. Based on droop control without interconnects, the power sharing problem can be effectively solved. A unified control strategy is adopted for islanded and grid-connected modes, achieving seamless switching between the two modes. Experimental results verify the effectiveness of the proposed control method.
